There is an epidemic of opioid overdoses associated with the use of illicit drugs such as heroin and the misuse of prescribed narcotic pain medications. Chronic use of opioids, even when under medical supervision, is associated with the development of tolerance and escalating risk of withdrawal. The opioid withdrawal syndrome is a serious medical problem that can become a major reason why addicts keep using opioid drugs. Currently the only treatments to prevent the withdrawal syndrome are either continuing to take opioids (including substitution drugs such as methadone) or symptomatic management of the many severe problems such as diarrhea, vomiting, dehydration, cramping, muscle pain, insomnia, irritability, and anxiety. Addicted individuals often prefer to continue drugs rather than face withdrawal. While the sudden reduction in mu opioid receptor activation is the proximal cause of withdrawal, surprisingly little is known about how downstream physiological processes contribute to the syndrome. Being ?dope-sick? has many attributes of a severe inflammatory state. Thus, it is not surprising that evidence is accumulating that both opioid tolerance and especially acute withdrawal produce a neuroinflammatory state that is a major contributor to the symptoms experienced. We propose the hypothesis that microglia, the resident immune cells in the brain, become activated during opioid withdrawal and that the inflammatory cascades mediated by these cells lead to much of the withdrawal syndrome. We will test this idea using two strategies. First, we will measure the RNAs in microglia cells that are actively being translated to make protein (as indicators of the biological pathways that are activated in these cells during withdrawal). We will investigate mice given escalating doses of morphine followed by precipitated withdrawal and then use RiboTag, a new technology for retrieving the RNA from specific cell types, to interrogate the sequential changes occurring during microglial activation from opioid withdrawal. Second, we will assess whether the withdrawal syndrome can be prevented by inhibiting microglial activation using engineered ?DREADD? receptors to inhibit microglia during withdrawal. These experiments will utilize state of the art transgenic strategies that are established in our lab and will allow us unprecedented precision in investigating and modulating microglia during acute opioid withdrawal. The ultimate goal of this proposal is to identify novel molecular targets in microglia that can prevent the inflammation associated with withdrawal, leading to the development of new treatments to mitigate opioid withdrawal. This will make withdrawal itself safer and potentially contribute to the motivation of addicted individuals to discontinue opioid use.